Electroplated palladium and palladium alloys are used in a variety of applications including deposition of protective coatings on decorative articles such as jewelry, watches, etc., in various containers and fixtures exposed to chemically corrosive liquids and gasses and in various electrical and electronic devices as a protective coating and electrical contact coating. Much of the motivation for use of palladium and palladium alloys in such applications is its lower cost compared to such traditionally-used metals as gold and platinum.
Early work on electroplating palladium and palladium alloys met with considerable difficulty. Often the electroplated palladium metal was not adherent, tended to be porous, often developed cracks and generally was quite brittle. Generally, such deposited palladium layers were not satisfactory either as electrical contact layers or as decorative coatings.
Investigation into the reason why electroplated palladium layers exhibit such poor quality soon revealed that this is due to the incorporation of hydrogen into the electroplated palladium layers. Hydrogen evolution often accompanies palladium electroplating because of the close proximity of the water electrolysis potential to the palladium electroplating potential. Incorporation of hydrogen into the electroplated palladium layers appeared to be responsible for the degraded properties of electroplated palladium. Indeed, many palladium electroplating processes appeared to work quite well under laboratory conditions where plating potential could be carefully controlled and the hydrogen evolution potential could be avoided and plating rates under these conditions are low. However, under commercial plating conditions, these processes proved unreliable either because the plating potential used was not precisely controlled or because higher plating rates required plating potentials that lead to the evolution of hydrogen during the electroplating process.
A major advance in palladium electroplating technology occurred with the discovery that certain palladium complex ions exhibited electroplating potentials far removed from the hydrogen evolution potential. The complexing agents involve certain aliphatic polyamines with best results obtained with 1,3 diamino propane. This work is described in U.S. Pat. No. 4,486,274 issued to J. A. Abys, et al on Dec. 4, 1984.
This discovery led to a major commercial effort in palladium electroplating. The process has been used extensively in the United States and throughout the world to electroplate palladium typically for electrical contact surfaces in various devices such as electrical connectors. It has generally been used in applications formerly requiring gold contact surfaces and has led to considerable cost savings because of the lower cost and lower density of palladium as compared to gold. Further development work has been done as described in such references as U.S. Pat. No. 4,468,296 issued to J. A. Abys et al on Aug. 28, 1984 (replenishment compound for a palladium electroplating process) and U.S. Pat. No. 4,493,754 issued to J. A. Abys et al on Jan. 15, 1985 (unique anode structure for use in palladium electroplating process). Often, the palladium layer of the contact surface is covered with a very thin layer of gold to improve wear and contact characteristics.
Because of the success of the palladium electroplating process involving aliphatic amines, further improvements both in the electroplating process and properties of the electroplated palladium have become desirable. In particular, cost reduction in the palladium electroplating process is desirable as is greater versatility in the choice of palladium electroplating species. Also, greater ductility and adhesion of the electroplated palladium is desirable particularly for relatively thick layers. Such thick layers of palladium metal and palladium alloys would be highly useful for devices where extended wear is required. Thickness of 25 .mu.m or more are of interest for a variety of applications.
In addition, it is highly desirable to have an inert palladium alloy substance that is not affected by the evolution of hydrogen, electroplates easily even at high electroplating rates and produces electroplated layers of sufficient ductility and thickness so as to be useful for fabricating articles by electroform processes.
A variety of references have disclosed palladium electroplating processes including U.S. Pat. No. 4,487,665 issued to K. B. Miscioscio et al on Dec. 11, 1984; U.S. Pat. No. 4,491,507 issued to G. Herklotz et al on Jan. 1, 1985 and U.S. Pat. No. 4,545,869 issued to I. Goldman on Oct. 5, 1985. The palladium tetra-ammine complex is used as the source of palladium in a number of palladium electroplating processes including those described in U.S. Pat. No. 4,622,110 issued to J. L. Martin et al on Nov. 11, 1986; U.S. Pat. No. 4,552,628 issued to J. Wilcox on Nov. 12, 1985 and U.S. Pat. No. 4,628,165 issued to F. I. Nobel on Dec. 9, 1986.